1. Field of the Invention
The present invention relates to a method of directly mounting a printing plate on a plate cylinder of a printing press for flexographic printing or letterpress printing and similar printing methods and the plate cylinder and register pins used in that method. More particularly, the invention relates to a method that facilitates direct mounting of a printing plate on a cylinder plate with accurate registry by forming small holes in the non-image area portion of the printing plate corresponding to the register holes made in the outer surface of the peripheral wall (hereunder referred to as simply the outer surface) of the plate cylinder and by bringing the small holes of the printing plate into registry with the register holes made in the plate cylinder. Furthermore, the invention provides for registrations both in a process of plate-making using a photopolymer plate and in a process of the mounting of the plate on the plate cylinder by using as a reference the register holes made in the outer surface of the plate cylinder, and small holes made in an original film and small holes made in the printing plate or in an unexposed photopolymer plate.
2. Description of Prior Art
A printing plate is conventionally mounted on a plate cylinder of a printing press by one of the following two methods in flexographic printing: (1) a carrier sheet typically made of a polyester film to which the printing plate is attached is wrapped around the plate cylinder; (2) the printing plate is directly mounted on the plate cylinder. The former method requires a great deal of skill or a special apparatus for mounting the printing plate on the carrier with accurate registry. In addition, registration is also required after the mounting of the carrier on the plate cylinder when the carrier is not wrapped with accurate registry. Furthermore, this method is not suitable for endless printing because the printing plate cannot be mounted on that part of the outer surface of the plate cylinder which is covered with the apparatus for fastening and removing the carrier.
The second method is compatible with continuous endless printing since it permits the printing plate to be mounted at a desired position on the plate cylinder. However, considerable difficulty is involved in accurately mounting the printing plate on the cylinder without a great deal of skill. In current practice, the cylinder is always removed from the printing press and the plate is precisely mounted on the cylinder with a special mounting/proofing machine. Furthermore, when the operator wishes to shift to another printing job, he must remove the first printing plate and mount the next printing plate by repeating the same procedures all over. If the operator wants to omit the steps of removing one printing plate and mounting another, he has to keep at hand as many cylinders as are necessary for performing the intended printing jobs, but this is undesirable for two reasons: firstly, a large space is required for storing the necessary number of cylinders, and secondly, it is expensive to purchase many cylinders. Further problems arise from the heavy weight of the cylinders such as removal of a cylinder from the printing press of remounting it on the press will not only present a great hazard to the operator but may also cause damage to associated parts such as gears for driving the plate cylinder.